The present invention relates to an apparatus and method useful for the vapor deposition of materials, and more particularly to an apparatus and method useful for the production of crystalline material using gas driven substrate rotation.
Materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices. These materials, however, may not be well suited for higher power and higher frequency applications because of their relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 eV for GaAs at room temperature) and/or relatively small breakdown voltages.
In view of increased interest in high power and high frequency applications and devices, attention has turned to wide bandgap semiconductor materials such as silicon carbide (SiC, with a bandgap of 2.996 eV for alpha SiC at room temperature) and the Group III nitrides, including gallium nitride (GaN, with a bandgap of 3.36 eV at room temperature). SiC and GaN also exhibit high breakdown fields of about 3 MV/cm, thus enabling such materials to withstand high power levels. In addition, GaN exhibits excellent electron-transport properties, which enables it to operate at high frequencies.
SiC and GaN materials can be prepared using vapor phase deposition techniques as known in the art, in which reactant gases, typically diluted in a carrier gas such as hydrogen, are introduced into a system to form a crystalline material in epitaxial layers upon an appropriate substrate. Various vapor phase deposition techniques have been further developed for the production of epitaxial layers of GaN in view of the difficulties associated with the vapor transport of gallium and other Group III components in non-organic form. For example, in MOCVD (metal-organic chemical vapor deposition) and OMVPE (organo-metallic vapor phase epitaxy), GaN is deposited from ammonia (as the nitrogen source) and organometallic compounds such as trimethyl gallium (as the Group III source) in the gas phase. GaN crystals can also be grown from the vapor phase using HVPE (hydride vapor phase epitaxy), in which hydrochloric acid reacts with metallic gallium, forming volatile gallium chloride that is carried to a growth surface and reacts with ammonia to form the GaN crystal.
Contamination of the growing crystal layers can be problematic in vapor phase deposition processes. Reactant gases can deposit on the side walls or top surface of the reactor chamber above the substrate or wafer growth surface. Thick deposits on upper surfaces can peel or flake off after they become too thick and fall onto upward facing substrates. The deposits can also react with gases introduced for subsequent layers and can redeposit as particles on the wafers during fabrication, thereby introducing impurities in the layers. In addition, hydrogen carrier gas can partially decompose at the growth temperatures required for such techniques, producing atomic hydrogen species. The atomic hydrogen species can attack the reactor surfaces, especially graphite, but also SiC coated reactor components. Atomic hydrogen species attack of reactor components can also produce particles that dislodge from the reactor, and the particles can fall onto, and thereby contaminate, the growing crystalline layers. Further, dopants introduced into the process to produce intentionally doped materials can adhere to the walls of a reactor. The reactor must be thoroughly cleaned following such a run, or the residual dopants can re-evaporate during subsequent runs and be incorporated into the growing crystal layers.
Achieving crystal layer uniformity can be also problematic. U.S. Patent Application Publication US 2004/0060518 illustrates an apparatus for MOCVD production of semiconductor materials that includes mechanical rotation of the substrate. See also Takayuki Arai et al., J. Crystal Growth 170 (1997) 88-91, which reports uniform crystal growth in a MOVPE system that includes mechanical rotation of multiple wafers.
The need for a mechanical feed through into the growth chamber in such systems, however, can result in various problems. The mechanical feed through can undesirably contribute to gas leakage from the reactor. Mechanical rotation can also be difficult to operate under the high processing temperatures typically employed in these systems. In addition, the materials requirements for the components of a mechanically rotated planetary system can lead to rapid wear and dust contamination, and further the tolerance requirements for such components can be difficult to meet.
Another disadvantage of many conventional vapor deposition reactors is that a large and non-uniform boundary layer thickness of hot air can form over the substrates as a result of heating the susceptor. During growth, heat from the susceptor can cause gases to rise and the boundary layer can extend to the top surface of the reactor chamber. Reactant gases are injected into the reactor chamber, typically through a top inlet. When the lower temperature reactor gases encounter the hot gases, heat convection can occur, which can cause turbulence within the reactor. This turbulence can result in non-uniform deposition of materials to the wafer. In addition, convection can contribute to layer contamination, for example, via dopant carryover from one grown layer to the next.
Accordingly, a need exists for an apparatus and method suitable for producing crystalline materials having reduced crystal contamination and substantially uniform crystal layer formation.